( x + 1 ) + ( x + 4 ) + ( x + 7 ) + ... + ( x + 28 ) = 155

x + 1 + x + 4 + x + 7 + ... + x + 28 = 155

( x + x + x + ... + x ) + ( 1 + 4 + 7 + ... + 28 ) = 155

có ( 28 - 1 ) : 3 + 1 = 10 ( số hạng)

=> có 10 x

1 + 4 + 7 + ... + 28 = ( 28 + 1 ) x 10 : 2 = 145

ta có:

( x + x + x +...+ x ) + 145 = 155

           10x             + 145 = 155

           10x                      = 155 - 145 = 10

              x                      = 10 : 10 = 1

vậy x = 1

a/ 7x⁵+21x³=7x³(x²+3)

b/4x³y³-8x²y²=4x²y²(xy-2)

c/3x(x-y)+2y(x-y)=(3x+2y)(x-y)=3x²-3xy+2xy-2y²=3x²-xy-2y²

d/5x(x-3)+2(3-x)=5x(x-3)-2(x-3)=(5x-2)(x-3)=5x²-15x-2x+6=5x²-17x+6

e/ 4x(x+7)-3(7+x)=(4x-3)(x+7)=4x²+28x-3x-21=4x²+25x-21

f/x²-9=(x+3)(x-3)

g/4x²-9=(2x+9)(2x-9)

3gio20phut=3 1/3gio

3 giờ nha

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 38 to 42.

(1) A small but growing group of scholars, evolutionary psychologists, are beginning to sketch the contours of the human mind as designed by natural selection. Some of them even anticipate the coming of a field called “mismatch theory”, which would study maladies resulting from contrasts between the modern environment and the “ancestral environment”. The one we were designed for.

(2) There is no shortage of such maladies to study. Rates of depression have been doubling in some industrial countries roughly every 10 years. Suicide is the third most common cause of death among young adults, after car wrecks and homicides.

(3) Evolutionary psychology is a long way from explaining all this with precision, but it is already shedding enough light to challenge some conventional wisdom. It suggests, for example, that the nostalgia for the nuclear family of the 1950s is in some way misguided that the model family of husband at work and wife at home is hardly a “natural” and the healthful living arrangement, especially for the wives. Moreover, the by gone lifestyles that do look fairly by commercialism. Perhaps the biggest surprise from evolutionary psychology is it depiction of the “animal” is us. Freud, and various thinkers since, saw “civilization” as an oppressive force that thwarts basic animal instincts and urges and transmutes them into psychopathology. However, evolutionary psychology suggests that a large threat to mental health may be the way civilization thwarts civility. There is a gentler, kinder side of human nature, and it seems increasingly to be a victim of repression in modern society.

The word “one” in line 4 refers to the _____.

A. mismatch theory

B. field

C. modern environment 

D. ancestral environment

does